The present invention relates to a manufacturing process of a functional film element, and, more particularly, to a manufacturing process of a functional film element suitably used for a fluid sensor, a temperature sensor, a fluid pump, a speaker, an actuator, a transducer or the like.
In recent years, there has been known a functional film element utilizing an energy conversion function of a functional film, in which a cavity is formed in a substrate, and a functional film actuator is mounted on the outer surface of the cavity, whereby the functional film actuator senses change of physical variable such as pressure or temperature occurring in the cavity, and outputs an electric signal corresponding to the variation of that physical variable. A functional film element utilizing such characteristics for converting variation of various physical variables into an electric signal is generally called a functional film sensor or a functional filter.
Contrary to the above, when an electric signal (voltage or current) is applied to such functional film element, the functional film actuator generates mechanical stress such as expansion and contraction, bending or vibration in the cavity according to the applied voltage or current, or generates heat so that it acts as a heater or the like for heating inside the cavity.
FIG. 4 shows an example of structure of a conventional functional film element 14 or a sensor. A functional film element 14 is constituted by providing through holes 2 and 3 to input the variable to be measured, forming in a ceramics substrate 9 a cavity 1 which is a measurement area, and integrally forming a functional film actuator 13 on a covering plate 6 for the external wall positioned opposite to the through holes 2 and 3 in the cavity 1. Here, the ceramics substrate 9 is integrally formed by laminating the covering plate 6 and a through hole plate 8 having the through holes 2 and 3 passing into the cavity 1, each of which is a very thin plate, with a window spacer plate 7 therebetween. The through holes are not limited to the through holes 2 and 3, but a through hole 4 or other through holes are provided depending on an object to increase contact points with the external environment.
The window spacer plate 7 is formed with a window 5 in such a manner that three through holes 2, 3 and 4 provided in the through hole plate 8 are formed in a longitudinal extent of the window 5 so that these holes are opened toward the window 5. In addition, a very thin covering plate 6 is laminated on a surface opposite to the side on which the through hole plate 8 of the window spacer plate 7 is laminated to cover and close the window 5, thereby forming the cavity 1 within the ceramics substrate 9.
Then, the ceramics substrate 9 is provided with the functional film actuator 13 on the outer surface of the covering plate 6 at a position corresponding to the cavity 1. Here, the functional film actuator 13 consists of a lower electrode 12, a functional film layer 11, and an upper electrode 10.
The conventional functional element 14 is arranged as above, in which, viewing the cavity 1 and the through holes 2, 3 and 4 in a plan view, the cavity 1 overlaps over the through holes 2, 3 and 4 as shown in FIG. 5. On the other hand, in recent years, as seen in a plan view of FIG. 2 showing a positional relationship between a window 29, and through holes 22, 23 and 24, there has been employed, to enhance detection accuracy of the functional film element, a shape in which through holes, for example, 22 and 23 out of those formed in the ceramics substrate are longitudinally elongated in a cavity 21 (window 29).
However, in actually manufacturing a functional film element to have the through holes 22 and 23 outwardly elongated from the outer edge of the cavity 21 (window 29), when ceramics green sheets are laminated and integrated under pressure, the through holes 22 and 23 provided through a window spacer plate 26 and a through hole plate 27 prevent transmission of pressure through each green sheet in the laminating direction, thereby making it difficult to apply sufficient pressing pressure, and sufficient adhesion cannot be obtained. Consequently, there is a problem in that a covering plate 25 and a window spacer plate 26 are separated at an adhered section between them facing the window 21 to generate a gap 41 after firing as shown in FIG. 3.
Generation of such gap 41 is unfavorable since when the functional film element is used as a pressure measuring sensor, air is left in the cavity when fluid to be measured for pressure is intended to be filled in the cavity to provide a pressure measurement error. In addition, when it is used as a temperature sensor, in measuring temperature of fluid such as liquid, air left in the gap frequently has thermal capacity different from that of the liquid, and the air causes delay of measurement accurately following temperature variation of the liquid. In addition, when it is used for a speaker, mechanical stress is imposed on the covering plate 25 due to vibration of the functional film element. This causes separation of the covering plate 25 or the like to proceed with the gap 41 as a starting point of breakage. As a result the speaker will break.